Being a photovoltaic element for energy conversion, a solar cell is able to convert light energy into electric energy under the irradiation of sunlight, with the aim of achieving power generation. An anti-reflection film is deposited on the surface of a solar cell to reduce light reflection on the surface of the cell and thus effectively improve the conversion efficiency of the cell. Meanwhile, the anti-reflection film can play a role of surface passivation and for the solar cell as well. Therefore, these two aspects should be taken into consideration comprehensively during the selection of the anti-reflection film material. The optical anti-reflection film material suitable for crystalline silicon solar cells includes silicon dioxide, silicon nitride or other thin film materials.
At present, anti-reflection films are grown mainly using PECVD the technical principle of which is as follows: with a low-temperature plasma serving as an energy source, a sample is heated up to a predetermined temperature by glow discharge or additional heating elements, and then a proper amount of reactive gases is fed in, followed by the fact that a solid-state thin film is formed on the surface of the sample after the reactive gases are subjected to a series of chemical reactions and plasma reactions. At present during the production of solar cells, ammonia and silane are generally used as the reactive gases, which form, upon the completion of reaction, a dark blue thin film in which silicon nitride plays a role of anti-reflection while hydrogen may exist for the purpose of body passivation. PECVD coating proposes high requirements on both temperature and vacuum degree. There are complicated gas paths due to the participation of several different gases in the reaction. Moreover, since the reactive gases are usually toxic, flammable and explosive, there exist potential safety hazards.
Researches have shown that a high voltage between a circuit existing in a crystalline PV module and a grounded metal frame thereof will result in continuous degradation of the photovoltaic performance of the module. There are a variety of mechanisms resulting in such degradation, for example, the phenomenon of ion migration in the packaging material for the cell of the module and in the material of the upper surface layer and lower surface layer of the module due to the effect of the above high voltage; the occurrence of hot carriers in the cell; reduction of the active layer of the cell owing to the charge redistribution; corrosion of related circuits, etc. These mechanisms resulting in such degradation are referred to as Potential Induced Degradation (PID), polarization, electrolytic corrosion and electrochemical corrosion. The majority of the above phenomena are most likely to occur under humid conditions, and their degrees of activity are related to the degree of humidity. Meanwhile, what is also related to the degradation phenomena mentioned above is the degree of contamination that conductive substances, acidic substances, alkaline substances and substances with ions cause to the surface of the module. In a practical application scenario, the PID phenomenon of the crystalline solar cell module has already been observed. Depending on the cell structure, different materials of other components and designs, the PID phenomenon may arise in a case when its circuit and the grounded metal frame create a positive voltage offset or in a case when its circuit and the grounded metal frame create a negative voltage offset. Related papers have stated that leak current, through a path formed by the packaging material (typically EVA and the upper surface of glass) and the module frame, is regarded as the main cause for the PID phenomenon. So far, it is actually not very clear how the leak current is formed. In general, an insulation system, which is formed after a cell is packaged, using the packaging material, is imperfect for the above leak current, and meanwhile, it is supposed that metal ions derived from soda-lime glass are the main current carrying medium of the abovementioned leak current having the PID effect. In addition, related papers have shown that, under actual application conditions, a certain period of time after the sun rises in the morning tends to be a time period during which the PID effect becomes relatively strong, the reason for which is that dew condensation (particularly dews in summer and autumn) may occur on the surface of the crystalline PV module without power generation through the whole night, and as a result of that the photovoltaic system, with its surface relatively humid, will encounter the abovementioned system voltage offset within the certain period of time after the sun rises in the morning.
At present, the solutions for solving PID focus mainly on three levels, i.e., solar cells, modules and solar systems or solar parks. The resistivity of solar cells, diffusion sheet resistance, anti-reaction film and other process qualities all have influences on the anti-PID performance of solar cells. For PV module industries, EVA, tempered glass, aluminum frames and other auxiliary materials also function as a bridge for the occurrence of potential induced degradation, so the improvement of such auxiliary materials is an effective way to solve potential induced degradation. As to solar systems and solar parks, potential induced degradation may be avoidable to some extent by improving inverters and grounding modes as well as by other such approaches.
With a life span up to 25 years, PV modules face a high likelihood of severe climate conditions. Serious PID may greatly decrease the power output of a power plant (degradation of over 90%), causing a grave damage to the interest of operators and power plant investors.